The present invention relates to a method and apparatus for die casting molten material, and more particularly to a method and apparatus for injecting a shot of molten material into the cavity of a die.
Die casting is a well known technique for forming articles from molten metal. Typically, the die casting apparatus includes a pair of die halves each formed with a void corresponding to a portion of the article to be cast. When the two die halves are brought together in proper alignment, their respective voids cooperate to form a die cavity corresponding to the shape of the article to be cast. Molten material is introduced into the die cavity and allowed to cure. Generally, the curing process involves cooling the molten material to allow it to solidify. Once the material is sufficiently cured, the die halves are opened and the cast article is removed.
In the past, a shot cylinder has been used to inject the molten metal into the die cavity. The cylinder includes a shot sleeve defining an internal bore and a transverse filling opening to allow molten metal to be poured into the sleeve. Upon actuation, the cylinder plunger both seals off the transverse opening and injects the molten metal into the die cavity. The open filling hole presents problems because air can be trapped in the sleeve. If overfilled, the shot sleeve can pressurize before the plunger seals the filling hole and molten material can spurt back through the filling hole as the plunger advances. Even if the shot sleeve is filled properly to prevent spurting, air may be trapped within the sleeve and injected with the molten material into the die, resulting in a porous casting.
A unique closed shot die casting arrangement that overcomes these problems is disclosed in U.S. Pat. No. 5,025,338 issued Apr. 27, 1993 to Shimmell. The '338 patent discloses a filling cylinder that intersects the shot sleeve and includes a reciprocating slide valve. After the internal bore of the shot sleeve has been filled with molten material, the slide valve is actuated to seal off the filling opening in the shot sleeve. Consequently, the shot sleeve is completely filled and sealed prior to the advancement of the plunger. While a significant advance in the art, the described closed shot die casting arrangement of the '338 patent requires relative complex machining in its manufacture. Further, the filling cylinder increases the profile of the shot sleeve arrangement so that it cannot be included on all desirable die casting equipment.